The invention pertains to color-television receiver circuits, in general, and to a phase-locked-loop (PLL) for the color subcarrier signal in a television receiver, in particular.
The present invention relates to a color-television receiver with at least one integrated circuit for processing the composite color signal digitized by means of a clock signal, the digital chrominance signal being locked to the chrominance-subcarrier frequency by means of a phase-locked loop. The integrated circuit includes a frequency- and phase-controllable chrominance-subcarrier oscillator forming part of the phase-locked loop and generating the clock signal for the digital processing. It also includes a color killer for inhibiting color reproduction; an alignment switch opening the phase-locked loop for frequency alignment, the alignment data being transferred over a data bus and stored in a frequency register, and a burst amplitude control circuit regulating the burst amplitude at the value of a burst reference and, if this value is not reached, inhibiting color reproduction via the color killer.
A color-television receiver of this kind is described in a data book by Intermetall Semiconductors ITT, "Digit 2000 VLSI Digital TV System", Edition 1984/5, pages 76 to 80. In this arrangement, the phase-locked loop can be opened, e.g., in the test shop, for alignment purposes by entering control and alignment data from outside over the data bus. During operation, the phase-locked loop remains closed. During reception of standard color-television signals, the phase-locked loop locks to the chrominance-subcarrier frequency contained in the burst signal, the frequency of the chrominance-subcarrier oscillator being four times the chrominance-subcarrier frequency. In the presence of nonstandard color-television signals, particularly if there is no or only a very small burst signal, as is the case, for example, with video games or home computers connected to the television receiver, but also during reception of pure monochrome signals without burst, the burst amplitude control circuit tries to amplify the noise or other components contained in the composite color signal to such a level that the phase-locked loop can lock to them. As a result, the phase-locked loop, having acquired lock with these components, leaves the desired frequency and, depending on the interfering frequency contained in the composite color signal, may be detuned up to the limit of the capture range.
If the color-television receiver is then switched to a channel on which a standard color-television signal is present, the phase-locked loop will not return to the phase-locked mode in some cases, particularly if the alignment frequency of the open phase-locked loop differs from the desired frequency due to component aging, temperature variations or imperfect alignment and, thus, takes up a portion of the capture and lock range of the phase-locked loop, thereby rendering the useable control range of the loop frequency unsymmetrical and narrowing it unilaterally. As a result of this loop behavior, the color reproduction is either inhibited or does not stabilize immediately but only after a considerably time.